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Experimental and Theoretical Investigation of Resonances in Low-Energy NO-H$_2$ collisions

The experimental characterization of scattering resonances in low energy collisions has proven to be a stringent test for quantum chemistry calculations. Previous measurements on the NO-H$_2$ system at energies down to $10$ cm$^{-1}$ challenged the most sophisticated calculations of potential energy surfaces available. In this report, we continue these investigations by measuring the scattering behavior of the NO-H$_2$ system in the previously unexplored $0.4 - 10$ cm$^{-1}$ region for the parity changing de-excitation channel of NO. We study state-specific inelastic collisions with both \textit{para}- and \textit{ortho}-H$_2$ in a crossed molecular beam experiment involving Stark deceleration and velocity map imaging. We are able to resolve resonance features in the measured integral and differential cross sections. Results are compared to predictions from two previously available potential energy surfaces and we are able to clearly discriminate between the two potentials. We furthermore identify the partial wave contributions to these resonances, and investigate the nature of the differences between collisions with \textit{para}- and \textit{ortho}-H$_2$. Additionally, we tune the energy spreads in the experiment to our advantage to probe scattering behavior at energies beyond our mean experimental limit.